The invention relates generally to interconnection and encapsulation of electronic components, in particular to interconnection and encapsulation methods for flip-chip integrated circuits, and specifically to material selection for interconnection and encapsulation of flip-chip integrated circuits.
Thermosetting resin compositions such as epoxy resins have been used as semiconductor device encapsulants for over 25 years as noted by reference to U.S. Pat. No. 3,449,641, granted Jun. 10, 1969.
U.S. Pat. No. 3,791,027, Angelo et al., describes epoxy fluxes for soldering. Angelo et al. teach that the fluxes may be formulated to be removable from the solder situs or may be formulated through cross-linking after the soldering process to form a thermoset epoxy polymer which remains at the solder joint and reinforces the strength of the solder joint.
Anhydride-cured epoxy resin encapsulants used in flip-chip manufacturing methods that are applied after electrical interconnection are described in U.S. Pat. Nos. 4,999,699, granted Mar. 12, 1991, and 5,250,848, granted Oct. 5, 1993.
The flip-chip method of attaching integrated circuits to substrate boards involves a series of metal solder bumps on the integrated circuit which form metallurgical interconnections with the metal bond sites on the board substrate. The active side of the integrated circuit is flipped upside down in order to make contact between the bumps on the chip and the metal bond sites on the substrate. An organic soldering flux is used to remove metal oxides and promote wetting of the solder when the assembly is heated above the temperature of the solder. This process is referred to as reflow soldering. The purpose of the flux is to clean the surface of the metals. The solder, or lower melting alloy, may be the composition of the board bond pads, of the bumps on the chip or both depending on the materials selected. Similarly, the higher melting alloy may be present on either the bond pad or the bumps on the chip. This process is derived from the controlled, collapse, chip, connect (C4) method developed by IBM in the-1960""s.
The reflow soldering operation provides a gap of 0.025 mm to 0.17 mm between the chip and the substrate. Although this small standoff height significantly enhances the electrical performance of the mounted flip-chip, the residue from the flux is difficult to remove from the narrow gap. Thus, no-clean fluxes, in which flux residues are not removed from the board after reflow soldering, are the flux type of choice for most flip-chip applications. These no-clean fluxes may be dispensed onto the metal bond sites on the board prior to chip placement. These liquid no-clean fluxes are formulated to contain more than 94% solvent which evaporates during the reflow process and flux activators which sublime during the reflow step. Thus, minimal amounts of residue remains on the board after reflow. These liquid fluxes, however, have difficulty in holding the chip to the board prior to reflow. The high solvent content of the flux causes the small integrated circuit to skew and misalign before peak soldering temperatures are reached. An additional problem arises from the volatility of many solvents used in these fluxes which blow the chips out of alignment during reflow. Although tackifying agents can be added to overcome these problems, the no-clean, low-residue requirement of the flux dictates a high solvent content which leads to alignment problems during reflow.
In order to maintain alignment of the chip to the board prior to reflow soldering, a viscous tacky flux may be applied to the bumps on the chip. This method involves dispensing the flux onto a rotating disk or drum then applying a blade above the rotating drum. Thus, a desired thickness of flux on the drum can be achieved by adjusting the height of the blade. The integrated circuit, containing solder bumps, is then dipped into the flux on the drum to a set depth. Using this method a desired amount of tacky flux is applied to the surface of the bumps only. The chip is then aligned and placed onto the substrate so that the bumps, which contain tacky flux, make contact with the appropriate metal bond sites. The tacky flux is formulated to contain a higher solids content which aids in the adhesion of the chip to the substrate prior to reflow. The tacky flux acts as a temporary glue to hold the chip in proper alignment during placement of the assembly into the reflow oven. The tacky flux contains less solvent which prevents the phenomenon of blowing the chips off the board during reflow commonly seen using liquid fluxes. Since only a small amount of flux is applied to the bumps, minimal residue remains on the board after soldering.
The tacky fluxes commonly used are the solder-paste flux vehicles used in no-clean surface mount processes. Although the formulations of no-clean solder paste flux vehicles vary, a typical composition contains 50% rosin, 40% solvent, 5-8% thickeners, and 2-5% flux activators such as organic acids and amines. The rosin, or a synthetic resin with similar characteristics, does not boil off during the reflow profile and is necessary to act as a carrier for flux agents at peak soldering temperatures. The residue which remains after soldering is typically rosin or a similar resin with any remaining ingredients such as decomposed organic acids, amines, thickeners, or other organic constituents of the solder paste. When these solder paste flux vehicles are used to solder flip-chip devices using the described drum flux method they provide desirable properties such as rolling on the drum, forming thin films and leaving minimal residue. The flip-chip assembly is formed by soldering the solder bumps of the integrated circuit to the appropriate metal bond sites of the organic substrate. The resulting flip-chip assembly has a gap between the integrated circuit and substrate. This gap is generally filled with an underfill encapsulant. The liquid underfill encapsulant is dispensed around the sides of the soldered flip-chip and allowed to flow under the assembly by capillary action. The purpose of the encapsulant is to relieve the thermomechanical stresses on the solder interconnections that are caused by the difference in thermal expansion coefficients between the silicon IC (CTE=15-20 ppm/xc2x0 C.) and the organic substrate (CTE=15-20 ppm/xc2x0 C.). Typical underfill encapsulants used in flip-chip assemblies are composed of epoxy resins, curing agents and inorganic fillers to yield a cross-linked thermosetting polymer when cured. The properties of the cured polymer, such as the CTE and elastic modulus, help relieve the thermomechanical stress on the solder joints during thermal cycle testing. Thermal cycling tests involve repeated exposure of the flip-chip assemblies to cycles of cold and hot environments. This repeated cycling induces thermal fatigue on the solder joints as the chip and organic substrate expand at different rates. A typical thermal cycle test involves repeated exposure of the flip-chip assembly to two different liquids at xe2x88x9255xc2x0 C. and +125xc2x0 C. with 10 minute dwell time at each temperature. Thus, the overall purpose of the underfill encapsulate is to enhance flip-chip assembly reliability by relieving the thermomechanical stress on the solder joints. Flip-chip assemblies on inorganic substrates, such as ceramic, do not generally use an underfill encapsulant as the OTE of ceramic closely matches that of the silicon IC.
Several process and material property characteristics dictate the material selection of the underfill encapsulant. First, the epoxy underfill encapsulant must flow quickly under the chip to achieve fast production cycle times. The viscosity, surface tension and particle size distributions can be optimized to achieve efficient flow under the chip during the encapsulation step. To further reduce the underfill time the substrate may be heated in order to reduce the viscosity of the uncured epoxy material. This heating significantly enhances the flow speed of the material. It is common to heat the surface of the substrate board to 70xc2x0 C. prior to dispense of the encapsulant in order to achieve this effect. Second, the epoxy underfill must cure quickly in order to achieve fast production cycle times. Typical underfill encapsulants are epoxy formulations designed to cure, i.e. form irreversible cross-linked structures, at temperatures above 150xc2x0 C. Finally, the epoxy underfill encapsulant must adhere strongly to both the chip and substrate during thermal cycling tests. If the epoxy pulls away, or delaminates, from either the chip or substrate surface, proper stress relief on the interconnects will not be achieved. The interface between the chip and the underfill is critical for proper thermal cycle reliability enhancement. It has been found that the interaction between the no-clean flux residue and the epoxy underfill encapsulant is critical to achieve maximum adhesion and proper flip-chip reliability enhancement.
As discussed, typical solderpaste flux compositions used as tacky fluxes for flip-chip contain rosin or a similar resin. After reflow soldering, a residue of rosin and other organic constituents of the flux remain on the substrate. Although these no-clean residues are benign to the assembly in terms of their corrosivity, these residues have been seen to adversely affect the adhesion of the epoxy underfill encapsulant. These rosin residues can be reheated and softened or even liquefied. Rosin softens at 55xc2x0 C. Since the underfill encapsulant is dispensed under the chip at temperatures of 70xc2x0 C., the epoxy underfill comes in contact with a liquid or softened residue. During cure, at temperatures at or above 150xc2x0 C., the epoxy is unable to properly adhere to the chip or substrate surface as the tacky flux residue is in a softened or liquefied state. The liquid or soft residue from the flux forms a barrier between the epoxy underfill and the surfaces of the chip and substrate. This may lead to early delamination from the chip surface poor adhesion of the underfill encapsulant.
This delamination of the encapsulant from the chip can be detected and measured using scanning acoustic microscopy (SAM). The SAM technique detects the presence of voids between the surface of the chip and the epoxy underfill. The SAM is used to first measure the total area of coverage then used to detect changes from this baseline value after thermal cycling tests.
In accordance with the present invention, there are provided tacky flux compositions for use in the soldering of flip-chip assemblies which contain in the most general terms 1.) an epoxy resin; 2.) a chemical cross-linking agent with fluxing properties; and 3.) a solvent. The tacky flux compositions of the present invention are also referred to herein as epoxy-based fluxes or epoxy-based fluxing agents. The compositions may be employed to solder flip-chip assemblies to substrates, such as organic substrates with high CTE values relative to the silicon IC, which require an underfill encapsulant to enhance reliability performance by reducing thermomechanical fatigue on the solder interconnects. The residue is present at minimum levels so as not to interfere with the underfill of the encapsulant, is designed to co-react with the underfill encapsulant and reduces delamination of the underfill from the substrate and chip during thermocycling reliability tests.
The present invention involves using an epoxy resin flux which does not interfere with solder melt and after the soldering step is partially cured to provide enhanced bonding at the interface of encapsulant and residue.
One aspect of the present invention relates to a protocol for selection of components for soldering flux for soldering flip-chip devices to circuit substrates, based on the ability of the solder flux components, when used in combination,
(1) to provide adequate flux activity to clean the surface metal oxides for a variety of solder alloy and metal bond compositions;
(2) to provide latency during the selected solder reflow profile; and
(3) to enhance bonding at the residue and underfill interface.